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Demixing and Tetratic Ordering in Some Binary Mixtures of Hard Superellipses

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Demixing and Tetratic Ordering in Some Binary Mixtures of Hard Superellipses Demixing and tetratic ordering in some binary mixtures of hard superellipses Sakine Mizani1, Péter Gurin2, Roohollah Aliabadi3, Hamdollah Salehi1 and Szabolcs Varga2 1Department of Physics, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran 2 Institute of Physics and Mechatronics, University of Pannonia, P.O. Box 158, Veszprém, H-8201, Hungary 3Department of Physics, Faculty of Science, Fasa University, 74617-81189 Fasa, Iran Abstract We examine the fluid phase behaviour of the binary mixture of hard superellipses using the scaled particle theory. The superellipse is a general two-dimensional convex object, which can be tuned between circular and rectangular shapes continuously at a given aspect ratio. We find that the shape of the particle affects strongly the stability of isotropic, nematic and tetratic phases even if the aspect ratios of both species are fixed. While the isotropic–isotropic demixing transition can be ruled out using the scaled particle theory, the first order isotropic- nematic and the nematic–nematic demixing transition can be stabilized with strong fractionation between the components. It is observed that the demixing tendency is strongest in small rectangle–large ellipse mixtures. Interestingly, it is possible to stabilize the tetratic order at lower densities in the mixture of hard squares and rectangles where the long rectangles form nematic phase, while the squares stay in tetratic order. 1 Introduction With the sudden development of the new colloidal and granular materials, the tailoring technique of the nano- and microparticles have created new shapes of particles (e.g. superballs, lenses, stars) which exhibit very interesting percolation, glass formation, jamming behaviour and phase transitions [1-3]. To understand the observed structural and phase behaviours, several geometrical properties (curvature, volume, contact distance, excluded volume) have to be determined since many macroscopic properties depends on the details of the shape and the size of the particles [4]. One of the simplest phase transition of rod-like particles, where the shape really matters is the nematic–smectic A phase transition of rod-like particles. In the system of hard particles, which consists of cylinders or spherocylinders, it is observed that the smectic A phase is stable if the particles are at least moderately elongated [5-6], while the smectic A phase is totally absent in the system of hard ellipsoids [7]. To understand this strange behaviour it was useful to introduce new geometrical models for the particles, which interpolate between the cylindrical and ellipsoidal shapes. In this regard the superellipsoidal [8-9] and the sphero- ellipsoidal [10] shapes have served new information about the importance of the roundness of the central part and the end parts of the particles, respectively. The other example is the suspension of colloidal superballs, where the shape is between spherical and cubic, exhibits richer phase behaviour than those of spheres or cubes. In the suspension of superballs, new solid phases are observed including a solid-solid transition between a plastic and a rhombohedral crystal [11]. The monolayer of colloidal superballs behaves very similarly because a phase transition between a hexagonal rotator crystal phase and a rhombic crystal phase emerges [12]. To explain this phase transition the simplest model is not the two- dimensional (2D) system of hard squares because the hard squares form only tetratic and square crystal phases [13-15]. Avendaño and Escobedo have performed Monte Carlo simulations with a rounded hard-square model, which interpolates between squares and disks, to examine the effect of roundness on the stability of crystal phases. They showed that the roundness of the vertices is crucial in the formation of rotor crystal phase [16]. With changing the shape of the 2D particles the simulation studies and the vibration experiments revealed the existence of new phases such as the triatic phase of triangles, the tetratic phase of squares and the hexatic phase of hexagons, which are intermediate between fluid and solid phases [17-22]. Interestingly the system of hard pentagons does not form intermediate phases between the fluid and solid phases 2 [23]. From these results it is evident that future studies require new geometrical models which can be tuned continuously between different shapes. The hard superellipse model is a good candidate to interpolate between two limiting shapes. If the lengths of both sides of the superellipse are equal, we get the 2D superball model, which interpolates between a disk and a square. The ellipse and the rectangle are also the limiting shapes of the superellipse, if the side lengths are unequal. The first milestone in the studies of hard superellipses was the exact determination of the maximal packing arrangement and the corresponding maximal packing fraction of hard superdisks [24]. Later, the jamming properties [25] and the kinetics of randomly packed superdisks [26] were also examined. These studies was extended to binary mixtures of hard superdisks to locate the jammed state [27]. Regarding the hard superellipses we are only aware of the contact point calculations [28] and the percolation threshold study of overlapping superellipses [29]. To study the phase behaviour of hard superellipses, the elaboration of overlap check between superellipses is the essential part in the simulations, while the excluded distances and areas are the key quantities in the mean-field theories such as the Onsager-theory and the scaled particle theory. The determination of these quantities are not trivial even for hard ellipses [30,31]. The issues of the excluded area calculation for convex and concave 2D objects are considered in two recent publications [32,33]. In our present study we show that the exclude area between two different superellipses having different sizes and shapes can be determined analytically. It is a well-known fact that the phase behaviour of mixtures is richer than those of one- component systems if the size and the shape of the components differ substantially. The competing different structures give rise to strong segregation and even demixing between identical phases [34]. In two dimensions the simplest binary mixture, which exhibit demixing transition is the nonadditive mixture of hard disks [35-41]. The positive nonadditivity is responsible for the fluid-fluid demixing transition, because it reduces the available room for both components upon mixing. It is showed that both symmetric (diameters of the components are the same) and non-symmetric hard disk mixtures can demix if the nonadditivity exceeds a minimum value [38,42,43]. The additive 3D hard body mixtures can exhibit isotropic-isotropic and nematic-nematic transitions if the constituting particles are rod-like [44-46]. Opposite to 3D rod-rod mixtures, only demixing transitions in nematic phase is found in binary mixtures of rod–like particles such as the rectangle-rectangle and ellipse–ellipse mixtures [47-50]. If the shape between the particles is not very different like in the mixture of hard disks and hard 3 ellipses, the demixing transition and the segregation are not present, but both rotational and translational glass transitions may occur [51]. In this work we examine the effects of varying shapes and sizes in the phase behavior of two-dimensional hard convex objects using the scaled particle theory. We choose the superellipse shape, which provides a bridge between circular and rectangular shapes. This shape allows us to consider new mixtures such as the ellipse–rectangle, where the shape of ellipse can be tuned into the direction of rectangle and vice versa for rectangle. We concentrate only on the stability of isotropic, nematic and tetratic ordering and search for possible phase transitions such as the isotropic–nematic, isotropic–tetratic and nematic–nematic ones. As the inputs of the theory are the areas of the particles and the excluded area between two particles, we calculate the area of the superellipse and derive an algorithm for the excluded area between two superellipses. To understand the shape dependence of the observed segregations (isotropic- nematic fractionation, nematic–nematic demixing) we make a link between the superellipse mixture and the nonadditive mixture of hard disks, where the nonadditivity parameter is the driving force of the fluid-fluid demixing. We finish our study with presenting a possible stabilization method of the low density tetratic order with mixing hard squares with long rod- like particles. Model In this work we examine the phase behaviour of some binary mixtures of hard 2D objects, where both the shape and size of the particles can be tuned easily. This 2D object is the hard superellipse with a semi-major axis length (a) and a semi-minor axis length (b). The equation of superellipse depends also on the deformation parameter (n) as follows n n x y 1 (1) a b where n 2 guaranties that the superellipse has a smooth curve and its tangent is well defined in its every point. For this reason, we deal only with this case in this paper. With increasing n, the shape of the particle can be tuned from the circular shape to the rectangular one. This opens the window to study a wider class of binary mixtures as the lengths and the shapes of both components can be varied independently. Using the diameter of component one ( 2b1 ) as a unit of the length, we have the following five independent molecular parameters: I) the aspect ratio 4 of component 1 ( k1 a 1/ b 1 ), II) the aspect ratio of component 2 ( k2 a 2/ b 2 ), III) the diameter ratiod b2/ b 1 and IV)-V) the exponents ( n1 and n2 ). Binary mixtures, which cannot be studied with the simple ellipse shape, are the followings: the square–square, square–ellipse, ellipse–rectangle and rectangle–rectangle mixtures. Note that this list is even longer if we replace one of the components or both components with superellipses having intermediate values of n1 and n2 which are between 2 (ellipse-limit) and infinity (rectangle-limit).
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